Functionalization of a substrate

ABSTRACT

A method of increasing a work function of an electrode is provided. The method comprises obtaining an electronegative species from a precursor using electromagnetic radiation and reacting a surface of the electrode with the electronegative species. An electrode comprising a functionalized substrate is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Filling under 35 U.S.C. 371from International Application No. PCT/CA2013/050291, filed on 15 Apr.2013, which published as WO 2013/152446 A1 on 17 Oct. 2013, whichapplication claims priority from U.S. patent application Ser. No.13/446,927 filed on Apr. 13, 2012, U.S. Provisional Patent ApplicationNo. 61/673,147 filed on Jul. 18, 2012, U.S. Provisional PatentApplication No. 61/806,855 filed on Mar. 30, 2013, and Canadian PatentApplication No. 2,774,591 filed on Apr. 13, 2012 the contents of each ofwhich are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The following relates generally to functionalization of a substrate.

BACKGROUND

Organic light emitting diodes (OLEDs) are becoming more widely used indisplays and other optoelectronic applications. Organic electronicdisplays typically consist of a matrix of OLEDs, each of which comprisesthin films of organic materials that emit light when excited by anelectric current. The organic thin films are typically sandwichedbetween an anode and a cathode, which provide an electric current to theorganic thin film to enable the film to emit light. In a display, thelight emitted by the organic thin film must exit the thin film andpenetrate through at least one of the electrodes to be visible to auser. Hence, at least one of the electrodes in the electrode paircomprises a transparent conductor such as a transparent conducting oxide(TCO).

Indium tin oxide (ITO) is the most commonly used TCO due to itstransparency and its high conductivity relative to other TCOs. ITO isused in various applications requiring transparency and conductivityincluding liquid crystal displays, plasma displays, photovoltaics,electronic ink displays, and OLED displays. ITO is typically depositedas a thin film on a transparent substrate such as glass.

In the context of OLEDs, an ITO layer is typically formed on atransparent substrate used as the anode. Holes are injected from theanode into a hole transport layer (HTL), which carries the holes to thelight emitting thin film layer. Concurrently, electrons are injected viathe cathode and are transported through the electron transport layer(ETL) and recombine with the holes in the light emitting thin film layerto release a photon. The photon emitted in the thin film layer may thenescape the thin film layer, pass through the HTL and exit the OLEDdevice through the ITO layer and the transparent substrate.

The energy required to inject holes from the anode is dependent on thehole injection barrier height. The hole injection barrier height dependson the difference between the work function of the anode and the highestoccupied molecular orbital (HOMO) of the adjacent organic layer. Thehole injection barrier of existing OLEDs is high but this can bemitigated by providing one or more intermediate organic layers. Eachorganic layer has a subsequently deeper HOMO level, enabling holes topass through a larger number of smaller injection barriers rather than asingle large injection barrier. However, each additional organic layerincreases the cost of the device and decreases the yield of themanufacturing process.

It is an object of the present invention to mitigate or obviate at leastone of the above disadvantages.

SUMMARY

In a first aspect, there is provided a method of increasing a workfunction of an electrode comprising obtaining an electronegative speciesfrom a precursor using electromagnetic radiation; and reacting a surfaceof the electrode with the electronegative species.

The electronegative species may be a halogen. The electromagneticradiation may have a wavelength of at least about 100 nm. Theelectromagnetic radiation may have a wavelength of less than about 400nm. The method may further comprise cleaning the surface of theelectrode. The electrode may be a transparent conducting oxide. Thetransparent conducting oxide may be ITO. The electronegative species maybe selected to obtain an electrode of a predetermined work function. Thesurface coverage of the species may be selected to obtain an electrodeof a predetermined work function. Up to about a monolayer of halogen maybe functionalized to the substrate. The halogen may be chlorine. Theprecursor may be a volatile liquid. The precursor may be a gas. Thesubstrate may be functionalized to increase its stability in air.

In another aspect, an electrode comprising a substrate functionalizedaccording to the above method is provided. An organic electronic devicecomprising the electrode is also provided.

In yet another aspect, there is provided the use of a system tochemically functionalize a substrate with a species, the systemcomprising a reaction chamber; a radiation emitter operable to emitelectromagnetic radiation into the reaction chamber; wherein thereaction chamber is operable to receive a precursor of the species and asubstrate; and wherein the electromagnetic radiation generates radicalsfrom the precursor of the species to chemically bond with the substrate.The radiation emitter may emit radiation having a wavelength of at leastabout 100 nm. The radiation emitter may emit radiation having awavelength of less than about 400 nm. In an example embodiment, theradiation emitter is external to the reaction chamber; and the reactionchamber is operable to at least partially transmit ultraviolet radiationfrom the radiation emitter.

In yet another aspect, there is provided a method of increasing a workfunction of an electrode comprising obtaining chlorine from a precursorusing a plasma; and reacting a surface of the electrode with thechlorine to form at least about 20% of a chlorine monolayer. In anexample embodiment, up to about a monolayer of chlorine may be reactedto the surface of the electrode. The substrate may comprise atransparent conducting oxide. The transparent conducting oxide may beITO. The surface coverage of the chlorine may be selected to obtain anelectrode of a predetermined work function.

In yet another aspect there is provided an electrode comprising asubstrate functionalized with at least about 20% of a monolayer ofhalogen. There is also provided an organic electronic device comprisingthe electrode. The organic electronic device may comprise an organiclight emitting diode. The organic light emitting diode may bephosphorescent. The organic light emitting diode may be fluorescent.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the appended drawings wherein:

FIG. 1 is a diagram illustrating a system in accordance with the presentinvention comprising a substrate which is functionalized withapproximately 0.5 of a monolayer of a species;

FIG. 2 is a diagram illustrating a system in accordance with the presentinvention wherein the substrate is functionalized with approximately 0.7of a monolayer of the species;

FIG. 3 is a diagram illustrating a system in accordance with the presentinvention wherein the substrate is functionalized with approximately onemonolayer of the species;

FIG. 4 is an X-Ray photoelectron spectroscopy graph showing that thebonding state of indium-chlorine bonds in InCl₃ is equivalent to thebonding state of indium-chlorine bonds in chlorine-functionalized ITO;

FIG. 5 is an X-Ray photoelectron spectroscopy graph showing therelationship between treatment time and chlorine functionalization of anITO surface;

FIG. 6 is an energy level diagram illustrating the work function of anexample ITO electrode;

FIG. 7 is an energy level diagram illustrating the work function of anexample chlorine-functionalized ITO electrode;

FIG. 8 is a representative chart showing the relationship between theapproximate surface coverage of chlorine on an ITO substrate and thework function of the ITO substrate;

FIG. 9 is an X-ray photoelectron spectroscopy graph comparing thebinding energy of various halogen-functionalized substrates;

FIG. 10 is a representative chart contrasting the change in workfunction over time in air for a chlorine functionalized ITO substrateand a bare ITO substrate;

FIG. 11 is a table showing the work function of various chlorinated andbare substrates after exposure to air;

FIG. 12 is a chart comparing the transmittance ofchlorine-functionalized ITO on a glass substrate to the transmittance ofa bare ITO electrode on the same glass substrate;

FIG. 13 is a chart showing a spectrum of the ultraviolet radiationemitter;

FIG. 14 is an energy level diagram of an example phosphorescent greenOLED construction comprising a bare ITO anode;

FIG. 15 is an energy level diagram of an example phosphorescent greenOLED construction comprising a chlorinated ITO anode;

FIG. 16 is a representative chart showing the relationship between thework function of a chlorine functionalized surface and the holeinjection barrier height into a hole transport layer;

FIG. 17 is an energy level diagram of an example phosphorescent greenOLED comprising a chlorine-functionalized anode;

FIG. 18 is a current-voltage chart showing a reduction in requireddriving voltage with increasing surface chlorination of an ITO anode;

FIG. 19 is a chart showing the relationship between current efficiencyand luminance of the OLED of FIG. 17 comprising an anode with amonolayer of chlorine;

FIG. 20 is a diagram showing the efficiency of the OLED of FIG. 17comprising a monolayer of chlorine on the ITO anode;

FIG. 21 is a table comparing the efficiency of the OLED of FIG. 17comprising a monolayer of chlorine on the ITO anode to phosphorescentgreen OLED devices in the prior art;

FIG. 22 is a chart showing the change in luminance over time for an OLEDcomprising a chlorine functionalized anode;

FIG. 23 is an energy level diagram of an example fluorescent green OLED;

FIG. 24 is a diagram showing the current-voltage characteristics of anexample fluorescent green OLED comprising a chlorine-functionalized ITOanode;

FIG. 25 is a diagram showing the efficiency of an example fluorescentgreen OLED comprising a chlorine-functionalized ITO anode;

FIG. 26 is a chart showing the current density with respect to electricfield for chlorine functionalized ITO anode with respect to other anodetypes;

FIG. 27 is a diagram illustrating an example plasma functionalizationapparatus in accordance with the present invention;

FIG. 28 is a diagram illustrating another example plasmafunctionalization apparatus in accordance with the present invention;and

FIG. 29 is an energy level diagram of an example phosphorescent greenOLED comprising an ITO anode functionalized with chlorine

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where considered appropriate, reference numerals may be repeated amongthe figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the example embodiments described herein.However, it will be understood by those of ordinary skill in the artthat the example embodiments described herein may be practised withoutthese specific details. In other instances, well-known methods,procedures and components have not been described in detail so as not toobscure the example embodiments described herein.

Also, the description is not to be considered as limiting the scope ofthe example embodiments described herein. For example, reference is madeto functionalizing a transparent conducting oxide (TCO) substrate. Itwill be appreciated that other substrates may be functionalized usingthe process described herein. Other non-transparent or non-conductingsubstrates may also be functionalized according to the process describedherein.

Provided herein is a method of functionalization of a substrate with aspecies. In particular, the functionalization of an electrode with anelectronegative species to increase the work function of the electrodeis provided. Also provided is a method of functionalizing TCO electrodesto achieve a higher work function without materially altering criticalproperties of the TCO electrode such as conductivity and devicestability. In one embodiment, the substrate is functionalized usingplasma disassociation of a precursor to release a reactive species, forexample, a halogen species. The halogen species is chemically reactedwith a substrate to increase the work function of the substrate.

Also provided is a substrate functionalized with up to about a monolayerof electronegative species. The electronegative species may be ahalogen. The halogen may be chlorine. The substrate may be a TCO. Anelectrode comprising a functionalized substrate is also provided. Thesubstrate may be functionalized to increase the work function of theelectrode. In an example embodiment, the substrate is functionalizedwith at least about 20 percent of a monolayer. A functionalization ofabout 20 percent may be a significant accomplishment. An organicelectronic device employing an electrode comprising a functionalizedsubstrate is also provided.

It has now been found that transparent conducting oxides includingindium tin oxide (ITO), which may also be referred to as tin-dopedindium oxide, may be directly used as an electrode in an organicelectronic device such as an organic light emitting diode (OLED). Thework function of ITO is approximately 4.7 eV in a vacuum. Because 4.7 eVrarely matches the highest occupied molecular orbital (HOMO) level ofmany common hole transporting materials, the use of ITO as an anodecauses a high hole injection barrier and poor operational stability ofthe device. It is well known that an anode having a work function thatis closer in energy to the HOMO level of the adjacent hole transportingorganic material would reduce the hole injection barrier, therebyreducing the required operating voltage and increasing the efficiencyand operation stability of organic electronic devices.

In the case of OLEDs, the active light emitting materials typically haveHOMO levels much greater than 4.7 eV. For example the HOMO level oftris(8-hydroxyquinolinato)aluminium (Alq₃), a fluorescent green lightemitting compound, is 5.75 eV. Although some organic light emittingmaterials may have HOMO levels closer to 4.7 eV, these materials aretypically doped into a host matrix that has a much higher HOMO levelthan 4.7 eV. Typically, holes must be injected into the HOMO of the hostin order for the dopant to emit light. For example, the HOMO level oftris(2-phenylpyridine)iridium(III) [Ir(ppy)₃], a phosphorescent greenlight emitting compound is 5.4 eV, but is commonly doped into a4,4′-bis(carbazol-9-yl)biphenyl (CBP) matrix. CBP has a HOMO level of6.1 eV, which is much greater than 4.7 eV. In particular, the HOMOlevels of the host materials used in phosphorescent OLEDs are about 6 eVor greater. Therefore, there exists a need for a transparent electrodehaving a work function that is greater, and preferably slightly greater,than the HOMO level of host materials used in OLEDs. In particular,there exists a need for a transparent electrode having a work functionof about 6 eV or higher.

One way to increase the work function of a TCO substrate is to clean thesurface of the substrate to remove contaminants. For example, thesurface of the TCO substrate may be cleaned using ultraviolet (UV) ozoneor O₂ plasma treatment. Plasma surface treatment and UV ozone surfacetreatment are effective in removing organic contaminants and may leaveelectronegative species on the surface of the TCO substrate. By way ofexample, UV ozone cleaning of the surface of an ITO substrate mayincrease its work function to about 5.0 eV. Cleaning the substrate maycause band bending at the surface of the substrate and an increase inthe surface dipole of the TCO due to electronegative oxygen species onthe surface of the substrate, thereby increasing the work function ofthe ITO substrate. Although reference is made to cleaning the TCOsubstrate using UV ozone or O₂ plasma, the substrate may be cleanedusing liquid cleaning methods, for example, using a detergent orsolvent.

It has been recognized that another way to raise the work function ofITO, which is an important TCO substrate, is to chemically treat the ITOsubstrate with an electronegative halogen, for example, fluorine. In theexample of an ITO substrate, a halogen may be reacted with indium atomsor tin atoms on the surface of the substrate to form up to anapproximate layer of indium halide. The process of reacting a surface ofthe TCO substrate with a halogen may be referred to as“functionalization”.

One way to chemically treat a TCO substrate with halogen is to react thesurface of the substrate with a halogen-containing acid (e.g.hydrochloric acid). A halogen gas may also be dissolved in a carrierliquid to be applied to the surface of a TCO electrode. However, theseprocesses are difficult to control, may etch the surface of thesubstrate, and may leave very little halogen functionalized to thesurface of the substrate. Hence, the substrate surface may becomerougher, and more contaminated, while the work function of the electrodemay not be sufficiently increased. Furthermore, the conductivity andtransparency of the substrate may be reduced using this process.Halogenation of a substrate using an elemental hydrogen containingsolution (e.g. HCl) may be combined with UV ozone or O₂ plasmatreatment.

The work function of a TCO substrate may also be increased using ahalogen containing plasma, which may cause a halide species to reactwith the surface of the TCO. For example, a fluorocarbon plasma such asCFH₃, an inorganic fluorine containing plasma such as SF₆, or a purehalogen plasma such as F₂ may be used. Multiple plasma gasses may beused in combination. A carrier gas may also be used, for example, Ar,He, or N₂.

Halogen-containing plasmas are typically used as standard reactive ionetching (RIE) industrial processes to dry etch substrates including TCOelectrodes. Therefore, halogen-containing plasmas typically etch thesurface of the substrate. This may decrease the conductivity of thesurface and may contaminate the surface with halocarbons. Halocarbonsare molecules comprising one or more carbon atoms covalently bonded toone or more halogen atoms (e.g. fluorine, chlorine, iodine, andbromine). The chemical bond between the contaminant and the substratedepends on the materials involved, type of plasma used and theprocessing conditions. The addition of an oxidant (e.g. O₂) may reducethe amount of deposited halocarbons and may also increase the rate atwhich the substrate is etched, negatively affecting other properties ofthe substrate. As is further described below, an example apparatus isprovided for functionalizing species to the surface of a substrate whilereducing the etching of the substrate.

It may be expected from the electronegativity of each of fluorine,chlorine, iodine, and bromine that fluorine functionalization providesthe highest increase in work function since it has the highestelectronegativity and therefore, would be expected to form the largestsurface dipole. Surprisingly, it has now been found that chlorinefunctionalized TCO's have a yet higher work function. This has beenconfirmed from density functional theory calculations and experimentalresults as measured by X-ray photoelectron spectroscopy (XPS) using anITO substrate that has been functionalized according to the processdescribed herein. Table 1 summarizes these results.

TABLE 1 Experimental and Theoretical Work Function of Functionalized ITOExperimental Work Halogen Functionalized Function (XPS) DensityFunctional to ITO Surface [eV] Theory Calculation [eV] fluorine 5.7 5.7chlorine 6.1 6.1 bromine 5.4 — iodine 5.2 —

Therefore, a chlorine-functionalized TCO may have a higher work functionrelative to TCO's functionalized with other halogens.

The above mentioned UV ozone and O₂ plasma cleaning treatments arereversible. For example, the surface of the cleaned TCO substrate may bere-contaminated, electronegative species on the surface of the TCO maydesorb, and the surface of the substrate is prone to hydrolysis. Theabove-described halogenation treatments offer greater stability than theUV ozone or O₂ plasma treatments, however, typical application of thesetreatments are prone to etching the surface of the TCO substrate.Furthermore, these halogenations treatments may affect other criticalproperties including the surface roughness, conductivity andtransparency of the TCO. Also, handling halogen-containing gases forplasma processes requires special safety precautions due to the toxicityand reactive nature of the materials involved.

The above-mentioned techniques may be unable to increase the workfunction of TCO substrates to a level enabling efficient injection ofholes into hole transporting organic materials with deep HOMO levels(e.g. 6 eV or greater). As a result, additional hole injection layers(HILs) and hole transport layers (HTLs) with HOMO levels between thework function of the TCO substrate and the HOMO level of the activeorganic layer are typically required in practical organic optoelectronicdevices to facilitate charge injection from the anode. For example, anumber of intermediate organic layers may be used, each having asubsequently deeper HOMO level. This enables holes to pass through alarger number of smaller injection barriers rather than a single largeinjection barrier. Each additional layer increases the cost of thedevice and decreases the yield of the manufacturing process.

Other methods to incorporate a TCO electrode with an insufficiently highwork function into a device involve coating the TCO with a high workfunction polymer (e.g. PEDOT), a self-assembled monolayer (SAM), or ametal oxide (e.g. WO₃). Such methods, however, may increase impedance,device complexity and fabrication cost, while introducing additionalproblems related to device stability.

The example embodiments described herein are, in one aspect, directed tothe functionalization of TCO thin films with halogens to modify theirwork function. In particular, example embodiments are described withreference to halogens and/or halocarbons released from ahalogen-containing precursor compound under ultraviolet radiation.However, it can be recognized that functionalization of other substratesusing the methods described herein falls within the scope of theinvention. In one embodiment, the substrate is functionalized usingplasma dissociation of a precursor to release an electronegativespecies, for example, a halogen. The halogen is chemically reacted witha substrate to increase the work function of the substrate. For example,functionalizing a substrate with a halogen using a halogen-containingplasma, and in particular, a chlorine-containing plasma, falls withinthe scope of the invention, as is further described below.

In another embodiment, a method of functionalizing the surface of asubstrate with a species is provided, wherein a precursor containing thespecies is dissociated using electromagnetic (EM) radiation. The speciesis then reacted with the substrate to increase the work function of thesubstrate. In particular, a TCO substrate may be functionalized with ahalogen by dissociating the halogen atom from a precursor using EMradiation. Any wavelength of electromagnetic radiation that breaks thebond between the species and the precursor may be used, however,ultraviolet (UV) radiation, has been found to be particularly effective.In particular, UV radiation having a wavelength of between 100 nm and400 nm was found to be effective. A catalyst may assist in breaking thebond between the halogen and the precursor. The catalyst may comprisethe chemical surface of the substrate.

The chemical bond between the species and the surface may increase thestability of the functionalized material in air, as will be furtherdescribed herein.

In some embodiments, and in examples that will be described herein, thehalogen-containing compound is a volatile halogen-containing organicprecursor. It will be appreciated that inorganic precursors may also beused. Organic precursors that release halogen atoms include halocarbons.The precursor may comprise two different halogens, for example, fluorineand chlorine. For example, the halogen-containing precursor may comprisea mixture of a fluoride and a chloride.

Example precursors include, for example, haloalkanes, haloakenes, andhaloaromatics. Common chlorinated precursors include chloromethane,dichloromethane tetrachloromethane, perchloroethylene,tetrachloroethylene, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane,carbon tetrachloride, chloroform, methylene chloride, trichloroethylene,methyl chloroform, 1,1,1-trichloroethane, 1,2,3-trichloropropane,ethylene dichloride, dichloropropane, dichlorobenzene, trichlorobenzene,propylene dichloride, 1,2-dichloroethylene, 1,1 dichloroethane, etc. Theprecursor may comprise a halogen-containing polymer such aspolytetrafluoroethylene (PTFE). The precursor may comprise metalhalides, for example, indium halides, zinc halides, and tin halides. Inan example, the metal halide species is preferably a constituentmetallic element of the substrate. A constituent metallic element willtypically not substantially alter the surface chemistry of the substrateif the metal component of the precursor remains on the surface of thesubstrate. For example, an indium halide may be used as ahalogen-containing species for ITO whereas a tin halide may be used as aprecursor for tin oxide substrates.

Upon functionalizing the substrate, residual contamination may beremoved by additional treatment with EM radiation of an appropriatewavelength. Contaminants may be removed using a UV ozone treatmentand/or using an appropriate plasma cleaning treatment, such as O₂plasma. The cleaning process is performed at a low energy to reduce thelikelihood of the surface of the substrate being etched. When using anorganic precursor, oxygen reacts with the remnants of organic precursormolecules to form volatile molecules (e.g. CO₂ and H₂O) which may beadvantageously flushed from the surface of the substrate. Volatilemolecules may also evaporate from the surface of the substrate. Hence,in some embodiments, organic precursors may leave less contamination incomparison with inorganic precursors after a cleaning step.

However, inorganic precursors may be used in the methods describedherein. Examples of these precursors include pure halogen gases,hydrogen halides, boron halides, sulphur halides, and phosphorushalides.

In some embodiments, the substrate may be functionalized with otherelements, for example, sulphur, boron, or phosphorus using appropriatevolatile precursors. For example, ammonium sulphide can be used tofunctionalize a substrate with sulphur. Other species that may befunctionalized to the surface of a substrate to alter the work functionmay be used.

The process of treating the substrate involves obtaining a transparentconducting (TC) substrate, for example, an ITO film deposited on glass.Other example TCO substrates include TCOs deposited on glass, such astin oxide, indium oxide, cadmium oxide, FTO, ZnO, NiO, MoO3, WO3, AuOx(oxidized gold), cadmium tin oxide (CTO), zinc tin oxide (ZTO), antimonytin oxide (ATO), aluminum zinc oxide (AZO), titanium zinc oxide (TZO),gallium zinc oxide (GZO), aluminum gallium zinc oxide (AGZO), indiumgallium zinc oxide (IGZO), gallium indium oxide (GIO), zinc indium oxide(ZIO), gallium indium tin oxide (GITO), zinc indium tin oxide (ZITO),titanium indium oxide (TIO), tin cadmium oxide (TCO), indium cadmiumoxide (ICO), zinc cadmium oxide (ZCO), aluminum cadmium oxide (ACO). Itwill be appreciated that other substrates including transparentconducting (TC) substrates may be used.

The TC substrate may be deposited on a transparent mechanical supportinglayer, for example, glass. The mechanical supporting layer may be rigid,flexible, planar, curved, or any other geometry that may befunctionalized using the method described herein.

The substrate may be comprised of a plurality of different layers. Forexample, the substrate may comprise multiple layers of different TCOs, ametal film on top of a TCO, a metal film sandwiched between two TCOlayers, or a thin layer of a high work function material such as atransition metal oxide on top of a metal or TCO layer. Various layers inthe substrate may be conducting, semiconducting, or insulating.

The substrate may comprise a plurality of layers of different metals,metal oxides, TCO's, polymers and carbon based materials. The electrodemay be a metal coated with a layer of metal oxide, including its nativemetal oxide. The electrode may be solid or porous. One or more layers ofthe substrate may comprise nano-material building blocks, for examplenano-particles, nano-rods, nano-tubes or other nano-scale materials. Oneor more layers of the substrate may comprise a composite of differentmaterials, for example nano-particles in a polymer matrix. One or morelayers of the substrate may comprise micron-scale particles.

The substrate may be patterned with nano-scale or micron-scale features,for example, features to enhance the out-coupling of light from anoptoelectronic device. One or more layers of the substrate may bepatterned. The substrate me be comprised of a plurality of layers withdifferent refractive index, for example to form a Bragg mirror orphotonic crystal.

The substrate may be transparent, semi-transparent, opaque orreflective. The substrate may include a mechanical support layer, suchas a piece of glass, flexible plastic, or semiconductor wafer. Thesubstrate and mechanical support layer may be the same material. Thesubstrate may be mechanically self-supporting, for example a metal foilor silicon wafer.

Although reference is made to functionalizing a substrate with ahalogen, it will be appreciated that the substrate may be functionalizedwith other species. For example, the substrate may be functionalizedwith a halocarbon to affect the surface energy of the functionalizedsurface. Typically, halocarbon treatments erode the surface of TCOsubstrates less than halogenations treatments, however, the equipmentrequired to perform the halocarbon treatments is specialized.Additionally, the conductivity of certain halocarbons is stronglydependent on processing conditions, and therefore, difficult to control.Even with precise control over processing conditions, the mostconductive halocarbons, for example, conductive fluorocarbons, are muchless conductive than many TCO's including ITO. However, EM dissociationof a halocarbon precursor to deposit a halocarbon film on a substratemay be achieved, as is described below with reference to FIG. 1.

Turning to FIG. 1, a system for functionalizing a substrate is provided.The system may comprise a reaction chamber 126, in which a substrate 104can be placed. A species may be deposited on a substrate 104 in thereaction chamber 126. The precursor compound 108 can be placed, or fedinto, the reaction chamber 126. The precursor compound 108 may be avolatile liquid or solid. The dissociation of the precursor 108 may takeplace in the vapour phase, liquid phase, or solid phase. Afunctionalization reaction with the surface of the substrate 104 maytake place on the surface of the substrate 104 in contact with thevapour phase. The precursor compound 108 may also be a gas, in whichcase no evaporation of a volatile precursor compound 108 is required torender the precursor compound 108 into the vapour phase. A gascomprising the precursor compound 108 may be provided into the reactionchamber 126 through a tube (not shown).

A radiation emitter 112 emits EM radiation into the chamber 126. Theradiation emitter 112 may emit UV radiation of, for example, between 100nm and 400 nm. The radiation emitter 112 may be located within thereaction chamber 126. The radiation emitter 126 may alternately beexternal to the reaction chamber 126 if the walls of the reactionchamber are at least partially transparent to the radiation.

In an example embodiment, the precursor 108 is applied directly to thesurface of the substrate 104, for example, in the form of a liquid orfine particulate (e.g. powder or nanoparticulate). The dissociationreaction of the precursor compound 108 and the subsequentfunctionalization of the substrate 104 reaction may proceed directly onthe surface of the substrate 104. The reaction may be catalyzed. Forexample, the reaction may be catalyzed by the surface of the substrate104. In an example embodiment, a catalyst is disposed in the system tofacilitate or enable the functionalization reaction.

Specifically, in the embodiment shown in FIG. 1, the precursor compound108 is a volatile liquid contained in an open reservoir 110. Theprecursor compound 108 evaporates into its vapour phase.

The substrate 104 may itself be deposited on a mechanical supportinglayer 102. For example, the substrate may comprise a TCO thin film (e.g.ITO) deposited on a glass substrate. The reaction chamber 126 isolatesthe substrate 104 from external contaminants and retains the precursorvapour and the reactive species in the vicinity of the substrate 104.

The radiation emitter 112 is operable to emit EM radiation 114 into thereaction chamber 126 to disassociate halogen species from thehalogen-containing precursor 108. The disassociation may be achieved inthe vapour phase, in the liquid phase (i.e. in the reservoir 110 or onthe surface of the walls of the reaction chamber 126), in the solidphase, or on the surface of a substrate. An example halogen containingvolatile precursor compound is dihalobenzene.

As the halogen species chemically bonds with the substrate, a monolayer106 a begins to form. As can be seen from FIG. 1, a partial monolayer106 a corresponding to approximately half of the surface of thesubstrate 104 has been formed. As will be explained in further detailbelow, the surface properties of the substrate 104 may be tuned based onthe surface coverage of the substrate 104 by the functionalizing species106 a.

As used herein, the term “monolayer” refers to a coating havingapproximately one layer of atoms. It is understood that a layer havingslightly more or less than a monolayer would be considered a monolayer.It is also understood that a monolayer containing impurities, forexample residual carbon, would be considered a monolayer.

Although the system of FIG. 1 is described in terms of functionalizing asubstrate with a halogen, in some embodiments, the species beingdeposited is a halocarbon. The halocarbon molecule may form a polymericstructure when functionalized to the surface of the substrate. Forexample, a fluorocarbon film may be deposited on the surface of thesubstrate. Fluorocarbon films comprising a C:F ratio controllably setbetween 1:3 and 3:1 have been achieved and confirmed via X-rayphotoelectron spectroscopy (XPS). Higher or lower ratios of carbon tohalogen are possible. XPS results have indicated the presence of CF₃,CF₂, CF, C—CF, and C—H bonds. Some species, for example, halocarbons maybe able to react to form multiple layers of a halocarbon film that maybe several manometers thick. The halocarbon film may be conductive ormay be insulating. The work function of the surface depends on theamount and type of halocarbon.

Other properties may be changed, including surface energy, to increaseor decrease the hydrophobicity of the surface. A surface may befunctionalized using a template to adjust the surface energy atparticular areas on the surface. A surface with a modified surfaceenergy may interact more favourably with certain species and resistinteraction with other species. For example, a hydrophobic surface wouldbead water while a hydrophilic surface could be wetted with water.Functionalizing particular areas of a surface may enable thefunctionalized regions to react with a species and the unfunctionalizedregions to be resistant to reaction and vice-versa. Although referenceis made to about half a monolayer being formed on the substrate 104,less than a monolayer may be formed. For example, at least about 20percent of a monolayer may be formed on the substrate.

Turning now to FIG. 2, the system of FIG. 1 is shown, however, thepartial monolayer 106 a in FIG. 1 has become more populated withchemically bonded species, as is shown by 106 b. The functionalizationreaction may be controlled by varying the wavelength of theelectromagnetic radiation used to dissociate the halogen from theprecursors, the intensity of the EM radiation, the temperature at whichthe reaction takes place, the precursor being used, the presence of anycatalysts, the substrate, and the halogen being functionalized to thesubstrate. The monolayer 106 b continues to form on the substrate 104 aslong as halogens continue to react with the substrate 104.

Referring now to FIG. 3, the substrate 104 is shown with a monolayer 106c formed on its surface. As described above, the monolayer 106 c mayhave imperfections (not shown). It may be desirable to cease thefunctionalization of the substrate 104 during the functionalizationprocess prior to forming a monolayer. The disassociation of precursors116 may be ceased by removing, blocking, or otherwise interruptingradiation from the radiation emitter 112. Once the release of halogenatoms from the precursors has ceased, the surface coverage remainssubstantially constant. The ability to stop the functionalizationreaction almost instantaneously enables control over the degree to whichthe substrate 104 is functionalized.

As is known, a functionalized substrate may include contaminants.Removing organic contaminants from the surface may increase the workfunction of the substrate 104. After functionalizing the desired portionof the substrate 104, the substrate 104 may be cleaned. Specifically,the substrate 104 may be cleaned to remove contaminants deposited duringthe functionalization reaction. For example, the contaminants maycomprise organic compounds originating from the precursor 108. In thecase of organic precursors, the contaminants may be reacted with UVgenerated ozone to produce volatile compounds which may be flushed fromthe reaction chamber 126.

The functionalization process may not significantly increase the surfaceroughness of the substrate 104. In an example embodiment, an ITOsubstrate was functionalized with chlorine. An atomic force microscope(AFM) was used to characterize the surface of a bare UV ozone treatedITO substrate and a chlorine-functionalized ITO substrate. The surfaceroughness, expressed in terms of the arithmetic mean value, R_(a), wasfound to be 2.2 nm for the bare surface and 1.9 nm after beingfunctionalized with a monolayer of chlorine atoms. It can be appreciatedthat the monolayer was not a perfect monolayer and there may be somevariability in coverage and contamination.

Referring now to FIG. 4, an XPS chart showing the 2p core-level energyspectrum of chlorine-functionalized ITO overlaid on the 2p core-levelspectrum of an InCl₃ reference is provided. The similarities between theInCl₃ curve and the chlorine-functionalized ITO curve suggest that theindium-chlorine bonds on the surface of the functionalized ITO substrateare in the same chemical state as the indium-chlorine bonds in InCl₃.

Turning now to FIG. 5, a chart of the approximate surfacefunctionalization (as estimated by the 2p peak intensity of chlorine)with respect to reaction time is provided. Several ITO substrates werefunctionalized with chlorine using the EM radiation dissociation method.The duration of the functionalization reaction of each substrate wasselected from between 0 and 10 minutes. XPS was used to measure theapproximate surface coverage of chlorine on the functionalizedsubstrates. As the reaction time of the functionalization processincreases from 0 to 10 minutes, there is a proportional increase in theintensity of the 2p peak, demonstrating that the functionalization ofsubstrate can be increased by increasing the reaction time. Conversely,with a shorter reaction time, the substrate is less functionalized,i.e., less than a monolayer is formed on the surface of the substrate.By selecting an appropriate duration of the functionalization reaction,the surface coverage may be tuned, for example, the surface coverage maybe tuned to a predetermined fraction of a monolayer.

FIG. 6 shows a band diagram of the work function of a standard ITOsubstrate with a bare surface. The work function of bare ITO isapproximately 4.7 eV (5 eV after cleaning), which is significantly lowerthan the approximately 6 eV that is desired to efficiently inject holesfrom the anode into the light emitting layer of typical organicelectronic devices.

Turning now to FIG. 7, an energy level diagram is shown for an ITOsubstrate that has been functionalized with a monolayer of chlorine isprovided. Each chlorine atom in the monolayer is chemically bonded to anindium atom in the ITO substrate, as was evidenced by the XPS chart ofFIG. 4, above. The work function at the surface of the functionalizedITO electrode is significantly higher than the work function of bare UVozone treated ITO. For example, the work function of ITO functionalizedwith a monolayer of chlorine may be approximately 6.1 eV in comparisonwith approximately ˜5 eV for bare, UV ozone treated ITO.

The increase in work function of the chlorinated substrate with respectto the bare substrate may be attributable to the surface dipole inducedby the chlorine atoms on the surface of the ITO. Therefore,functionalizing species increase the work function of the ITO inproportion to their dipole moment with the surface of the substrate. Adesired increase in the work function of an electrode can be obtained byselecting an appropriate functionalization species. Surprisingly, as wasdescribed above, chlorine achieves the highest dipole despite being lesselectronegative than fluorine. Density functional theory calculationsindicate that the In—Cl bond length is greater than the In—F bondlength, resulting in a larger net dipole moment for chlorine incomparison with fluorine.

The work function of the TCO substrate may be tuned within a range bycontrolling the reaction time, for example, by interrupting or blockingradiation 114 from the radiation emitter 112. The concentration of theprecursor compound may also be selected to tune the range. For example,by depositing less than a monolayer on the surface of a substrate, thework function may be set to be lower than the work function of asubstrate that has been functionalized with a full monolayer of speciesbut higher that of a bare substrate surface.

Referring to FIG. 8, a chart illustrating the relationship between workfunction and surface coverage of chlorine on an ITO substrate asapproximated from the chlorine 2p core-level XPS results is provided.The work function is approximately linearly related to the surfacefunctionalization of the ITO substrate. It will be appreciated that thechart of FIG. 8 is a rough approximation and is a representation of therelationship only.

By way of example, a functionalization of about 15% of a monolayercorresponds to a work function of approximately 5.65 eV. Afunctionalization of approximately 95% of a monolayer corresponds to awork function of approximately 6.15 eV. Hence, the work function may betuned depending on the application by functionalizing the surface withup to a monolayer. When a higher work function is desired, for example,above 6.1 eV, the surface may be functionalized with about a monolayerof chlorine. As stated above, it will be appreciated that the monolayer,or portions of the monolayer, may be imperfect.

In the context of OLEDs, ITO is commonly used as an anode. The workfunction of a functionalized ITO anode may be tuned to match the HOMOlevel of the organic hole transporting material, as is further describedbelow.

Referring to FIG. 9, XPS core-level spectra of ITO functionalized withiodine, bromine, and fluorine are provided. As was described above,chlorine induces the largest dipoles in the halogen-indium bond on afunctionalized ITO surface, thereby providing the maximum increase inwork function relative to ITO functionalized with other halogens.

As can be seen in Table 1 below, the functionalization of various TCOsurfaces is possible. UPS refers to ultraviolet photoelectronspectroscopy.

TABLE 2 Experimental work function from XPS/UPS of VariousFunctionalized Substrates Work Function of Work Function ofFunctionalized Clean Substrate Functionalized Substrate Species [eV]Substrate [eV] ITO chlorine 4.7 6.1 FTO fluorine 4.9 5.6 ZnO chlorine4.7 5.3 Au chlorine 5.2 6.2

In addition to increasing the work function of the electrode, halogenfunctionalization increases the stability of the work function of theelectrode relative to that of a bare UV ozone or O₂ plasma treated TCOelectrode. Turning to FIG. 10, a chart showing the stability of the workfunction of an ITO substrate functionalized with a monolayer of chlorineis compared with a bare ITO substrate in the presence of air. Thesurface of the bare substrate was treated with UV ozone for 15 minutes.As can be seen from FIG. 10, the work function of the bare electrodedrops by approximately 0.1 eV over about three hours in the presence ofair. In contrast, there is no substantial change in the work function ofthe functionalized substrate. This demonstrates the increased stabilityprovided to functionalized substrates. This may be advantageous in aproduction environment, as a functionalized substrate may be left inatmospheric conditions for a period of time without impacting the workfunction of the substrate. Higher stability may enable substrates to bestored in air, rather than in a vacuum or under an inert gas. Thestability of the functionalized substrate depends on the ambientenvironment including the ambient temperature and humidity.

Turning to FIG. 11, a table is provided showing the work function ofvarious substrates after being exposed to air for a period. It can beappreciated that under the same conditions and after exposure to air,the work function of the functionalized substrate is substantiallyhigher than the work function of the bare substrate under the sameconditions.

Referring now to FIG. 12, a chart is provided to illustrate that thetransmittance characteristics of a chlorine-functionalized ITO substrateare not substantially inferior to the transmittance characteristics of abare ITO substrate. The ITO layers were deposited on transparentsubstrates. As can be seen from the chart, the transmittance curves arevery similar over a wide range of wavelengths. Importantly, the curvesare almost indistinguishable over the visible spectrum, illustratingthat a chlorine-functionalized ITO anode may be used in an organicoptoelectronic device with no increase in the attenuation of lighttransmitted through the anode relative to that of a bare ITO anode.

Turning to FIG. 13, a spectrum of the ultraviolet lamp used tofunctionalize the ITO substrates in the examples above is provided.Specifically, the spectrum corresponds to that of a PL16-110 PhotoSurface Processing Chamber (Sen Lights™). The wavelength correspondingto the cut-off wavelength of Pyrex™ glass, which may be used as areaction chamber, is also provided.

The conductivity of a chlorine-functionalized ITO substrate is also notsubstantially inferior to the conductivity of a bare ITO substrate. Asmeasured with a 4-point probe the sheet resistance of an examplechlorine-functionalized ITO substrate is 18.2 Ohms per square, comparedto 18.1 Ohms per square for a bare ITO substrate.

One application of a transparent conducting substrate, for example, anITO substrate that has been functionalized to have a high work function,is the use of the substrate in an organic electronic device.Functionalizing the surface of an ITO substrate with a halogen speciesto increase the work function of the ITO substrate can reduce the holeinjection barrier. Reducing the hole injection barrier improves theefficiency of hole injection in an OLED, thereby decreasing the amountof voltage required to induce a current in the device.

It will be appreciated that although a functionalized TCO substrate isshown in an example OLED construction, other OLED constructions may usefunctionalized TCO substrates. Furthermore, other types of electronicdevices may comprise functionalized TCO substrates.

FIG. 14 shows an example energy diagram of an embodiment of an OLEDusing a transparent conducting substrate from the prior art. An ITOlayer 1280 is typically formed on a transparent substrate used as theanode. Holes are injected from the anode 1280 into a hole injectionlayer (HIL) 1282, then to a hole transport layer (HTL) 1284, through anelectron blocking layer (EBL) 1286 and into to the light emitting thinfilm layer 1292. Concurrently, electrons are injected via the cathode1298 and are transported through the electron transport layer (ETL)1296, through the hole blocking layer (HBL) 1294, and recombine withholes in the light emitting thin film layer to release photons. Thephotons emitted in the thin film layer may then escape through ITO layer1280 and any transparent substrate supporting the ITO layer 1280. Aghost line 1290 is provided to show the relative work function of achlorine-functionalized ITO layer, which is significantly better alignedwith the emitting layer.

FIG. 15 is an energy level diagram for a simplified phosphorescent OLEDcomprising a chlorine functionalized ITO electrode 1380. At lowerbarrier heights, holes can be injected more efficiently from the anode.As can be seen from the diagram, the height of the hole injectionbarrier, which is dependent on the difference between the HOMO of theemitting layer and the work function of the ITO electrode 1380, isrelatively low for the chlorine functionalized electrode. This lowerhole injection barrier enables the electrode to inject directly into thehost 1284, thereby enabling the host and the HTL 1283 to be the samematerial. Since the chlorine-functionalized anode is closely alignedwith the HOMO level of the HTL 1283, there is no need for the HIL layer1282. In contrast, a bare ITO electrode 1280 has a high injectionbarrier, making it inefficient to inject holes without the intermediateHIL, as was shown in FIG. 14. If the HTL and ETL are selected to haveappropriate energy levels, as understood by one skilled in the art, theEBL and HBL may also be eliminated.

Referring now to FIG. 16, a UPS chart showing the relationship betweenthe work function of the anode and the barrier height for holes in anOLED device is provided. It can be seen that increasing the workfunction of the electrode using halogen functionalization reduces thehole injection barrier height.

In an example embodiment, a chlorine-functionalized ITO anode wasprepared for use in a phosphorescent green bottom emitting OLED. An OLEDcomprising a chlorine-functionalized ITO anode and another OLEDcomprising a bare UV ozone treated ITO anode were fabricated in a KurtJ. Lesker LUMINOS™ cluster tool with a base pressure of 10⁻⁸ Torr oncommercially patterned ITO coated glass (25 mm×25 mm). ITO substrateswere ultrasonically cleaned with a standard regiment of Alconox™dissolved in deionized (DI) water, acetone, and methanol. The ITOsubstrates were then treated using UV ozone treatment for 3 minutes in aPL16-110 Photo Surface Processing Chamber (Sen Lights).

Chlorine-functionalized ITO was prepared by functionalizing the surfaceof the ITO substrate for 10 minutes according to the method described inFIG. 1 and in a Pyrex™ Petri dish with 0.2 ml 1,2-dichlorobenzene as theprecursor compound. A Pyrex™ reservoir was used as the chamber and theUV source was located outside of the chamber. A transmission spectrum ofPyrex™ is provided in FIG. 24a and the spectrum of the UV lamp isprovided in FIG. 24b . Once the functionalization reaction was complete,the ITO substrate was treated in UV ozone for 3 minutes.

The organic layers and the LiF cathode were thermally deposited fromalumina crucibles in dedicated organic chamber. The Al layer wasdeposited in a separate dedicated metal deposition chamber from a boronnitride crucible without breaking vacuum. All layers were patternedusing stainless steel shadow masks to define the device structure. Theactive area for all devices was 2 mm².

The standard device structure is as follows: anode/CBP (35nm)/CBP:Ir(ppy)₂(acac) (15 nm, 8%)/TPBi (65 nm)/LiF (1 nm)/Al (100 nm),where Ir(ppy)₂(acac) is bis(2-phenylpyridine)(acetylacetonate)iridium(III), and TPBi is2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole).

An energy level diagram 1200 of the example phosphorescent OLEDstructure is provided in FIG. 17. The chlorine-functionalized ITO anode1202 has a significantly higher work function than a bare UV ozonetreated ITO anode 1206. Hence, the chlorine-functionalized anode isbetter able to inject holes into the CBP layer 1204, as the HOMO levelof the CBP layer 1204 is well aligned with the work function of thechlorine-functionalized ITO anode. The Ir(ppy)2(acac) layer 1208 may bedoped into the CBP layer 1204. The TPBi layer 1210 is in electricalcommunication with the LiF/Al cathode layer 1212 and the Ir(ppy)2(acac)layer 1208.

FIG. 18 is a diagram showing the current-voltage characteristics of theexample device of FIG. 17. As can be seen, as the treatment timeincreases to a point where a monolayer is formed, the voltage requiredto drive current decreases. Hence, if a monolayer of chlorine isfunctionalized to the surface of the ITO anode used in the example OLEDdevice, the voltage required to operate the OLED may be significantlyreduced. As can be seen from FIG. 18, the voltage may be reduced byapproximately 4 V at an equivalent current density.

FIG. 19 is a chart of the current efficiency of the example OLED deviceof FIG. 17 comprising a chlorine-functionalized anode with respect tothe luminance being output from an OLED reference device from the priorart. Specifically, the OLED device comprises a UV ozone treated anodewith the structure: anode/PEDOT:PSS (5 nm)/α-NPD (35nm)/CBP:Ir(ppy)2(acac) (15 nm, 8%)/TPBi (65 nm)/LiF (1 nm)/Al (100 nm),where α-NPD is N, N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine. Itwill be appreciated that the PEDOT:PSS (5 nm)/α-NPD (35 nm) layers inthe reference device are required to inject holes into theCBP:Ir(ppy)2(acac) emission layer from the bare UV ozone treated ITOanode. It can be seen from FIG. 19 that the chlorine-functionalizedanode increases the current efficiency with respect to the referenceOLED comprising a bare UV ozone treated electrode. In particular, athigh luminance, the OLED comprising the functionalized anode issignificantly more efficient.

Turning to FIG. 20, the current efficiency and external quantumefficiency (EQE) of the phosphorescent OLEDs comprising a chlorinefunctionalized anode is provided. The phosphorescent OLED has a highmaximum current efficiency of 93.5 cd/A at 400 cd/m², which correspondsto a maximum EQE of 24.7%. At 10,000 cd/m² the current efficiency andEQE are still relatively high at 79.6 cd/A and 21% respectively. Turningto FIG. 21, the example OLED of FIG. 17 comprising a chlorinated ITOanode is compared with devices constructed using methods found in theprior art. As can be seen, the OLED comprising thechlorine-functionalized anode may be constructed to be significantlymore simple in terms of device layers and materials and may furtherexhibit a significantly higher external quantum efficiency.

Referring now to FIG. 22, a chart is provided showing the change inluminance measured in vacuum for the example OLED with the structure:electrode/CuPc (25 nm)/α-NPD (45 nm)/CBP:Ir(ppy)₂(acac) (15 nm, 8%)/TPBi(10 nm)/Alq₃ (45 nm)/LiF (1 nm)/Al (100 nm), where CuPc is copperphthalocyanine. As can be seen, the luminance of an OLED comprising anITO anode that has been functionalized with chlorine is higher than anOLED comprising a bare UV ozone treated ITO anode after being inoperation for several hours. This demonstrates that the OLED comprisingan ITO anode maintains a relatively higher luminance over time.

In another example embodiment, a fluorescent green OLED was fabricatedfollowing the same procedure as for the phosphorescent OLED outlinedabove. The standard device structure of the OLED is as follows:anode/CBP (50 nm)/Alq₃:C545T (30 nm, 1%)/Alq₃ (15 nm)/LiF (1 nm)/Al (100nm), where CBP is 4,4′-bis(carbazol-9-yl)biphenyl, Alq₃ istris(8-hydroxy-quinolinato)aluminium, and C545T is2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-Benzothiazolyl)quinolizino[9,9a,1gh]coumarin.

An energy level diagram 900 of the fluorescent OLED structure isprovided in FIG. 23. Numeral 902 refers to the chlorine-functionalizedITO anode, which has a significantly higher work function than the bareITO anode 906. Hence, the chlorine-functionalized anode 902 is betterable to inject holes into the CBP 904, as the HOMO level of the CBP 904is well aligned with the work function of the chlorine functionalizedITO anode 902. The OLED further comprises an Alq₃:C545T layer 908 and anAlq₃ layer 910, which is in communication with the LiF/Al cathode 912.

The HOMO level of the CBP layer in contact with the anode isapproximately 6.1 eV. The work function of the functionalized anode isapproximately 6.1 eV and the work function of the bare anode isapproximately 5.0 eV, after being treated with ozone, as measured byultraviolet photoelectron spectroscopy (UPS). The work function of thebare anode is too low to efficiently inject holes into the OLED whereasthe work function of the functionalized anode is more aligned with theHOMO level of the CBP layer.

FIG. 24 is a chart showing the current-voltage characteristics of afluorescent green OLED comprising a chlorine-functionalized ITO anodewith respect to a fluorescent green OLED of identical constructioncomprising a bare UV ozone treated ITO anode. As can be seen from FIG.24, the voltage required to achieve a particular current density issignificantly lower for the OLED comprising the chlorine-functionalizedanode.

Specifically, the current density of the OLED device comprising thefunctionalized electrode dramatically increases with a driving voltageof more than 6 volts. At 10 volts, the current density of the OLEDcomprising the chlorine-functionalized ITO anode is approximately 300mA/cm². In contrast, the current density of the OLED comprising the bareITO electrode is insignificant. The higher current density of the OLEDcomprising the chlorine-functionalized ITO anode demonstrates that thehigher work function enables more efficient injection of holes intoorganic hole transporting materials with deep HOMO levels.

A major advantage of aligning the work function of the ITO anode withthe HOMO of the CBP layer is that the power efficiency of the OLED isincreased; that is to say, the light output per unit of electrical inputin increased. Referring to FIG. 25, a chart showing the current andpower efficiencies of the OLED devices discussed above is provided. Thedevice with bare ITO anode has a lower power efficiency and a lowercurrent efficiency due to the poor injection of holes from bare ITOanode into the deep 6.1 eV HOMO of CBP. The device withchlorine-functionalized ITO anode has a much higher efficiency, with amaximum current efficiency at a luminance of approximately 1000 cd/m² of23 cd/A versus the approximately 18cd/A for the bare ITO anode.

Similarly, the power efficiency of the OLED comprising achlorine-functionalized ITO anode is approximately 12 lm/W at aluminance of 1000 cd/m², whereas the power efficiency of the OLEDcomprising the bare ITO anode is approximately 5 lm/W at a luminance of1000 cd/m². The increased power efficiency suggests that the chlorinefunctionalization of the ITO anode has a significant effect on powerefficiency.

Given the improved alignment of the work function of the chlorinated ITOanode and the HOMO of the CBP layer, it may be possible to forego theseveral HILs and HTLs that are typically required in such a deviceconstruction without an unacceptable loss of efficiency. Foregoing therequirement for HTLs is advantageous, as the number of processing stepsrequired to construct the OLED device may be reduced, thereby increasingthe manufacturing yield of OLED devices and reducing costs associatedwith their production.

The halogen functionalized electrode in the examples above contributeslittle series impedance to the device. For example, achlorine-functionalized ITO anode was prepared for use in asingle-carrier hole-only organic device. The structure of the device isas follows: anode/α-NPD (536 nm)/Ag (50 nm). A first device comprising abare UV ozone treated anode was compared to a second device having a UVozone treated ITO anode coated with 1 nm of vacuum deposited MoO₃, and athird device comprising a chlorine-functionalized ITO anode. The holeinjection barrier height between the anode and the α-NPD organic layerwas measured for each device using UPS. The hole injection barrierheight was 0.6 eV for bare UV ozone treated ITO, 0.45 eV for UV ozonetreated ITO coated with 1 nm of vacuum deposited MoO₃ and 0.45 eV forchlorine-functionalized ITO. The performance of the device with the UVozone treated ITO coated with 1 nm MoO₃ may initially be expected to bethe same as the device with the chlorine-functionalized ITO since thebarrier height for holes is the same for both devices.

FIG. 26 is a diagram showing the current-voltage characteristics of theexample single-carrier hole-only organic devices described above. Thecurrent density at a given voltage is highest for the device with thechlorine-functionalized ITO anode. The device with the UV ozone treatedITO anode coated with 1 nm MoO₃ is nevertheless exhibiting a highercurrent density at any given voltage than the device with the bare UVozone treated ITO anode due its lower hole injection barrier height.Unexpectedly, the current density for the device with thechlorine-functionalized ITO anode is higher at a given voltage than forthe device with the UV ozone treated ITO anode coated with 1 nm MoO₃,despite the same barrier height for holes. The lower current density inthe device with the UV ozone treated ITO anode coated with 1 nm MoO₃shows that the MoO3 layer introduces a series impedance into the device.

As was described above, a substrate may also be functionalized using aplasma. FIG. 27 is a plasma system for functionalizing a substrate. Thesystem comprises a reaction chamber 2608, which is grounded 2612. Thesystem may comprise a plurality of rods 2620 which support a substratesupport 2626 upon which the substrate 2652 may be placed. The substrate2652 is placed in electrical communication with the substrate support2626. The substrate 2652 is deposited on a non-conductive mechanicalsupport 2650, for example, glass. A high energy plasma shield 2624 isalso provided. The plasma shield 2624 may also be supported by the rods2620. The plasma shield 2624 is also grounded.

A radio frequency (RF) power source 2610 provides power to the poweredelectrode 2611. The reaction chamber 2608 comprises an inlet 2614through which a gas comprising a precursor may be pumped and an outlet2616 through which the vacuum chamber can be evacuated by a vacuum pump.The precursor may be a liquid or a gas. When the powered electrode 1611is powered by an RF power source 2610, a plasma is generated between thepowered electrode 1611 and the grounded portions of the system includingthe reaction chamber 2608. In particular, the highest energy plasma isgenerated in the region of the highest electric field, which may bebetween the powered electrode 2611 and the plasma shield 2624. However,plasma may also be generated elsewhere in the chamber 2608.

It will be appreciated that various plasma methods for generatinghalogen-containing plasmas may be used, including glow discharged basedplasmas. It will also be appreciated that plasma may be used incombination with UV light treatment to functionalize a substrate. Forexample, dichlorobenzene diluted in argon gas may be used withultra-violet light or with radio frequency electromagnetic radiation tofunctionalize a substrate with a halogen species.

The plasma causes the dissociation of any precursors in the reactionchamber. The dissociated precursors may then react with the surface ofthe substrate 2652 to begin to form a monolayer 2654. As is well known,particles in the plasma may have a substantial kinetic energy. Theplasma shield 2624 prevents the plasma having the highest kinetic energyfrom directly impinging on the surface of the substrate 2652, therebyreducing the etching effects on the substrate 2652.

By way of example, the chamber may be pumped down to about 250 mTorr and1,2-dichlorobenzene may be leaked in as a precursor for an ITOsubstrate. The substrate may be treated for approximately 5 minutes.Etching of the substrate was minimized due to the positioning of thesubstrate behind the plasma shield 2624. The functionalized substratemay be cleaned to remove residual contaminates.

Another example plasma system for functionalizing a substrate isprovided in FIG. 28. In the example of FIG. 28, the RF power source 2610is connected to the substrate support 2626, and hence, to the substrate2652 itself. A grounded electrode 2711 is positioned in a parallelarrangement with the substrate 2652. When the power source 2610 isactivated, a plasma is generated between the grounded electrode 2711 andthe substrate 2652. As outlined above, the plasma causes thedissociation of any precursors in the reaction chamber. The dissociatedprecursors may then react with the surface of the substrate 2652 tobegin to form a monolayer 2654. To mitigate any etching effects of theplasma, the gas comprising the precursor may be diluted with a carriergas, as is outlined below.

It will be appreciated that various known precursor-containing gases maybe used. For example, other halogen-containing precursors orfluorocarbon-containing precursors may be used. Example precursorsinclude bromine, chlorine, tri-chloroethane, dichlorobenzene,haloalkanes, haloakenes, and haloaromatics. Common chlorinatedprecursors include chloromethane, dichloromethane, trichloromethane,tetrachloroethane, perchloroethylene, tetrachloroethylene,1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, carbon tetrachloride,chloroform, methylene chloride, trichloroethylene, methyl chloroform,1,1,1-trichloroethane, 1,2,3-trichloropropane, ethylene dichloride,dichloropropane, dichlorobenzene, propylene dichloride,1,2-dichloroethylene, 1,1 dichloroethane, etc. The precursor may alsocomprise a halogen-containing polymer. Inorganic precursors may also beused. Examples of inorganic precursors include pure halogen gases,hydrogen halides, boron halides, sulphur halides, and phosphorushalides.

A difference in ionization energy between the precursor and carrier gascan be advantageously used to affect the amount of halogen radicals orions generated using electromagnetic radiation. For example, in aplasma-based process using RF electromagnetic radiation chlorine gas mayionize more readily than argon gas. Therefore, although theconcentration of chlorine in argon may be relatively low, for example,1%, the concentration of active ionized chlorine species in the plasmamay be much higher.

The precursor may comprise a mixture of various halogen-containingprecursors. An oxidizing agent, for example oxygen, may also be added tothe diluted precursor mixture to increase the removal of carbonimpurities from the surface of the substrate.

Plasma functionalization of the substrate may be employed in procedureswhereby a UV treatment may be damaging to the substrate, or otherwiseadversely affect the substrate. For example, in some cases, thin filmtransistors (TFTs) on the substrate may be damaged by UV radiation.Specifically, UV radiation may degrade a semiconductor substrateincluding silicon substrates and indium gallium zinc oxide (IGZO)substrates.

Functionalization of a substrate using plasma may be advantageous, asthe plasma treatment is typically conducted under vacuum, as are manyother steps of OLED fabrication. As such, the functionalization processcan be conducted in the same line of operations as other steps of OLEDfabrication without requiring a substantial increase in pressure,thereby reducing the time required to perform the functionalizationstep. Furthermore, existing plasma equipment may be used tofunctionalize substrates. Existing plasma equipment may be used even forlarge substrates including so-called “Generation 8” substrates, whichare 2.2 m by 2.5 m.

In another example, the plasma may make use of a carrier gas to reducethe concentration of the halogen-containing species. A gas having alower concentration of halogen-containing species may be safer to handleand may further reduce the etching effects of the plasma. For example,pure chlorine gas is extremely toxic and corrosive and is typicallystored in pressurized cylinders. Chlorine gas that has been diluted witha carrier gas, for example, 1% chlorine gas in an argon carrier, is lesstoxic and corrosive.

By way of example, the carrier gas may comprise a noble gas, forexample, argon. Other example carrier gases include helium, neon,krypton, and xenon. The carrier gas may comprise a mixture of thesegases in various proportions.

The carrier gas may comprise up to about 99.9% of the total gas volume.For example, the halogen-containing precursor may be introduced atconcentrations of about 0.1%, 1%, 5%, or 10% with one or more carriergases comprising the balance. The concentration of thehalogen-containing precursor may be selected depending on the desiredprocessing time, fractional coverage of the substrate, reactivity of thehalogen and substrate, or various other processing parameters. Lowerconcentrations of halogen-containing precursor plasmas havecomparatively lower etching rates than higher concentrations of the samespecies.

For example, the concentration of the gas containing thehalogen-containing precursor may be 5% in a 95% carrier gas. In aspecific example, a 5% dichlorobenzene and 95% argon mixture may beused.

By way of example, an ITO substrate coated on a glass sheet wasfunctionalized with chlorine gas diluted in an argon carrier gas at aconcentration of about 1%, as out below.

ITO substrates were cleaned with detergent, acetone and methanol andloaded into a commercial Advanced Energy reactive ion etching system.The processing chamber was pumped down to a base pressure of 10-2 Torrusing a scroll-pump. Chlorine gas diluted in argon at 1% concentrationwas leaked into the processing chamber using a mass flow controller to apressure of between 1 and 10 mTorr. A forward radio frequency power ofabout 50 W at 13.56 MHz was applied to the processing chamber, resultingthe formation of a chlorine and argon plasma. The substrate was treatedfor 10 seconds. It will be appreciated that other treatment times,operating pressures and plasma powers may be used. In this example, arelatively low power was chosen to minimize etching of the sample.

The work function of the treated sample was measured using x-rayphotoelectron spectroscopy and was found to be >6.0 eV. The Cl 2pcore-level of chlorine on the surface of the sample suggests theformation of In—Cl bonds. The surface roughness of the sample measuredusing atomic force microscopy was found to be about 2 nm, which issubstantially the same as the bare substrate prior to the plasmatreatment.

Organic light emitting diodes with the structure of CBP (35nm)/CBP:Ir(ppy)2(acac) (8%, 15 nm)/TPBi (65 nm)/LiF (132 nm)/Al (100 nm)were fabricated on the plasma functionalized ITO substrates. These OLEDsexhibited a relatively high external quantum efficiency of 24%,demonstrating that such processes can be used to prepare electrodes fororganic optoelectronic devices, such as OLEDs. It will be appreciatedthat although ITO was used, other substrates may be functionalized withhalogens using a similar process, for example, other transparentconducting oxides (TCOs), metal oxides or metals may be functionalized.

A process similar to that described above may also be used forfunctionalization using other halogens or for functionalizing a surfacewith chalcogenides, for example sulfur from a sulfur containingprecursor diluted in a carrier gas. Functionalizing a substrate withchlorine using RIE is also possible.

As described above, metal halides may be used as precursors. Exampleprocesses for functionalizing an ITO substrate using metal halideprecursors are set out below. Specifically, an ITO substrate wasfunctionalized with chlorine from a metal chloride precursor in thevapour phase in Example 1, in the liquid phase in Example 2, and usingan organic precursor in Example 3.

Example 1

An ITO coated glass substrate was ultrasonically cleaned with a standardregiment of Alconox™ dissolved in deionized (DI) water, acetone, andmethanol. The ITO substrates were then treated using UV ozone treatmentfor 3 minutes in a PL16-110 Photo Surface Processing Chamber (SenLights).

The surface of the ITO substrate was exposed to InCl₃ in the vapourphase in a vacuum chamber by subliming InCl₃ powder from an aluminacrucible. The exposure was equivalent to depositing about a 5 Å thicklayer of InCl₃ on the surface of the substrate as determined by a quartzcrystal microbalance.

The ITO substrate was treated using UV ozone treatment for 3 minutes ina PL16-110 Photo Surface Processing Chamber.

Example 2

An ITO coated glass substrate was ultrasonically cleaned with a standardregiment of Alconox™ dissolved in deionized (DI) water, acetone, andmethanol. The ITO substrates were then treated using UV ozone treatmentfor 3 minutes in a PL16-110 Photo Surface Processing Chamber.

The surface of the ITO substrate was exposed to a dilute solution ofInCl₃ dissolved in ethanol by spin-coating the solution onto the surfaceof the substrate at 2000 rpm.

The ITO substrate was treated using UV ozone treatment for 3 minutes ina PL16-110 Photo Surface Processing Chamber.

Example 3

An ITO coated glass substrate was ultrasonically cleaned with a standardregiment of Alconox™ dissolved in deionized (DI) water, acetone, andmethanol. The ITO substrates were then treated using UV ozone treatmentfor 3 minutes in a PL16-110 Photo Surface Processing Chamber.

The surface of the ITO substrate was treated with 1,2-dichlorobenzene(DCB) vapour in a closed Pyrex® Petri dish under UV irradiation in aPL16-110 Photo Surface Processing Chamber for 10 minutes.

The ITO substrate was treated using UV ozone treatment for 3 minutes ina PL16-110 Photo Surface Processing Chamber.

The chemical composition and work function of all of the substrates werecharacterized using x-ray photoelectron spectroscopy (XPS) in a PHI 5500Multi-Technique System using monochromatic Al K_(α) (hv=1486.7 eV). Anx-ray photoelectron spectroscopy (XPS) spectrum is provided in FIG. 28to show the chemical state of the chlorine bonded to the surface usingthe three example procedures set out above. Specifically, the XPSspectrum shows the 2p core-level energy spectrum of the three differentchlorine functionalized ITO substrates, functionalized using InCl₃vapour (Example 1), InCl₃ in solution (Example 2), and DCB vapour usingUV light. The similarities between the Cl 2p core-level spectrum for thethree different chlorine functionalized substrates suggests that thesurface chemical state of the three samples is similar.

As such, FIG. 28 indicates that the surface of ITO can be reacted withInCl₃ in the vapour phase and InCl₃ dissolved in solution tofunctionalize the surface with chlorine. The work function of each ofthe three chlorine functionalized ITO substrates can be estimated to be6.05±0.05 eV. Since the surface chemical state and work function of theITO substrates functionalized with InCl₃ vapour and InCl₃ dissolved insolution are similar to the ITO substrate functionalized usingdichlorobenzene and UV light.

It will be appreciated that the substrates functionalized using a metalhalide are expected to function in the same way when used in a device,for example an OLED, as the ITO substrates described above, given theirsimilar chemical structure.

It can be appreciated that potential applications of organicoptoelectronic devices comprising substrates functionalized according tothe method described herein comprise organic photovoltaics, OLEDs,organic thin film transistors, and biomedical devices. It will beappreciated that although reference is made to organic electronicdevices comprising TCO functionalized electrodes, inorganic electronicdevices may comprise functionalized TCO electrodes. For example, LCDelectrodes may be functionalized using the process as described herein.

Other potential applications of substrates functionalized according tothe methods described above comprise functionalizing a substrate toadjust surface energy, and templating growth of materials on a substratethat has been selectively functionalized.

It will be appreciated that although examples were provided disclosingthe functionalization of TCO substrates, other types of substrates maybe functionalized. For example, metals, polymers (including conductivepolymers), and ceramics may be functionalized with a species using theprocess as described herein.

Although example methods of functionalizing substrates are providedabove, it will be appreciated that substrates may be functionalizedusing methods other than those described. For example, it will beappreciated that electrodes comprising TCO substrates may befunctionalized with up to about a monolayer of a halogen species usingmethods other than those described above.

Although the above has been described with reference to certain specificexample embodiments, various modifications thereof will be apparent tothose skilled in the art without departing from the scope of the claimsappended hereto.

We claim:
 1. A method for increasing the work function of a substrate,the method comprising: providing a mixture comprising a precursor and acarrier gas; applying energy to the mixture to generate a plasma toobtain an electronegative species from the precursor; and reacting thesubstrate with the electronegative species to bond the electronegativespecies to the substrate, wherein the concentration of the precursor inthe mixture is less than the concentration of the carrier gas to reduceetching of the substrate when reacting the substrate with theelectronegative species, wherein the substrate is a transparentconducting oxide that is indium tin oxide, and wherein theelectronegative species is chlorine.
 2. The method of claim 1, whereinthe carrier gas is selected based on a difference between the ionizationenergy of the carrier gas and the ionization energy of the precursor. 3.The method of claim 1, wherein the carrier gas is an inert gas or anoble gas.
 4. The method of claim 1, wherein the precursor is selectedfrom the group consisting of chlorine, tri-chloroethane,dichlorobenzene, chloromethane, dichloromethane, trichloromethane,tetrachloroethane, perchloroethylene, tetrachloroethylene,1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, carbon tetrachloride,chloroform, methylene chloride, trichloroethylene, methyl chloroform,1,1,1-trichloroethane, 1,2,3-trichloropropane, ethylene dichloride,dichloropropane, dichlorobenzene, propylene dichloride,1,2-dichloroethylene, 1,1 dichloroethane, and metal halides.
 5. Themethod of claim 1, wherein the coverage of the electronegative specieson the substrate is selected to obtain a predetermined work function. 6.The method of claim 1, wherein at least 5% of a monolayer of theelectronegative species is bonded to the substrate.
 7. The method ofclaim 6, wherein at least 25% of a monolayer is bonded to the substrate.8. The method of claim 7, wherein at least 80% of a monolayer is bondedto the substrate.
 9. The method of claim 1, wherein the plasma durationand power are selected to reduce etching of the substrate.
 10. Themethod of claim 2, wherein the substrate is shielded from the plasma toreduce etching of the substrate.
 11. A method of increasing the workfunction of a substrate, the method comprising depositing indiumchloride molecules on the surface of the substrate.
 12. The method ofclaim 11, wherein the deposition is vacuum-based.
 13. The method ofclaim 3, wherein the concentration of the precursor is less than 10%.14. The method of claim 13, wherein the concentration of the precursoris less than 5%.
 15. The method of claim 14, wherein the concentrationof the precursor is less than 1%.
 16. The method of claim 15, whereinthe concentration of the precursor is less than 0.1%.